With 100 billion neurons and trillions of synapses, your brain spins neural webs of staggering complexity. It propels you to breathe, twitch, and butter toast, and yet we remain largely ignorant of how the brain does even these simple tasks—let alone how it stirs up consciousness.

To peer inside this three-pound lump of flesh, scientists manipulate a subtle trait of the body—its susceptibility to magnetic fields. Magnetic resonance imaging (MRI) has exposed the brain in stunning anatomical detail, and a sibling method, functional magnetic resonance imaging (fMRI), has offered insight into the mind at work. Here we explore how neuroscientists are using these methods to reveal new dimensions of the human brain.